Radiation therapy, also known as radiotherapy, is the medical use of ionization radiation as part of cancer treatment. Such treatment includes controlling or killing malignant cells. The amount of radiation used in radiation therapy is measured in Gray (Gy) and varies based on the type and stage of cancer being treated. Therefore, doctors plan the type and amount of radiation given to a patient based on the type of cancer in addition to considering the patient's health, age, weight and other factors.
Currently, in radiotherapy clinics, advanced treatment planning and delivery methods include increasing the radiation dose to reach the maximum tolerance that a normal tissue endures. To achieve such advanced treatment, there is an increase in demand for radiation methods that provide highly precise localization and motion control both before and during radiation treatment. Image-guided radiation therapy (IGRT) is critically important for the delivery of highly conformal radiation doses. In addition, advanced treatment techniques such as online and offline adaptive radiotherapy cannot be implemented without the motion information provided by online imaging modalities.
Computed tomography (CT) has become an important volumetric imaging modality for IGRT. CT imaging provides a transverse image of an object. Conventional fan beam CT uses a point x-ray source and a linear detector array. The detector array may have one or more detector rows. With a single rotation, one or more image slices can be reconstructed using computer algorithms. Different CT techniques may be used for the different treatment modalities. In some examples, a megavoltage fan beam CT (MVCT) is used for a helical tomotherapy system. In other examples, a megavoltage cone beam CT (MV-CBCT) is used. The major drawbacks of MVCT are lack of soft tissue contrast and high imaging dose due to the high x-ray energy. One improvement made to the MV-CBCT system is the use of a low atomic number target, such as carbon to shift the bremsstrahlung spectrum to the lower energy range. Another improvement is the development of CT on-rail systems, in which a diagnostic helical CT scanner is installed in the treatment room for IGRT purposes. During the IGRT treatment, the bed where the patient lies is rotated by an angle, usually 180 degrees, to align with the path of the rails on which the CT scanner is mounted and then rotated back to the treatment position after imaging is complete. While this system provides superior image quality, it is not a popular imaging modality mainly because it is inconvenient for the patient and lacks intra-treatment imaging capability (the organ movement within one treatment on a given day).
Kilovoltage (kV) cone-beam CT (CBCT) is an online volumetric imaging modality used for LINAC-based radiation treatments. The kV CBCT system includes a radiographic kV x-ray tube and a flat panel imager (FPI). The kV apparatus is installed on an additional structure that is orthogonal to the MV treatment beam. The kV CBCT system is convenient to use, allows the patient to remain in the same position for both imaging and treatment, and provides better soft tissue contrast than the megavoltage modalities. However, despite these advantages, specifically the convenience to the patient, the performance of the kV CBCT system is still not ideal. Excessive scatter photons are a major problem for CBCT, and the performance of the FPI is inferior to that of helical CT scanners. Another, but less significant problem is that CBCT suffers from approximate reconstruction artifacts at large cone angles because the circular trajectory of the system does not meet the data sufficiency condition. Because of its inferior image quality, clinical uses of CBCT are mostly restricted to localization in IGRT treatments. The inferior image quality also limits its use for advanced IGRT treatment techniques, such as online and offline adaptive radiotherapy, in which soft tissue contrast is important for deformable image registration and segmentation. Furthermore, the reconstruction artifacts and excessive scatter in CBCT make it difficult to accurately calibrate CT numbers, which poses a challenge to the use of CBCT images for dose calculation.
In addition to volumetric imaging, real-time imaging is also desirable in order to monitor intra-fraction motion, which is the organ movement during radiation delivery. While the fluoroscopic imaging function of CBCT may be used for real-time tracking, a single kV beam positioned orthogonally to the megavoltage (MV) beam is not an optimal configuration. This configuration is insensitive to motion that is orthogonal to the MV beam and may result in geometric miss during treatment delivery. MV portal imaging may be used, but in many situations, the image quality produced by the MV beam is insufficient to detect relevant anatomical features or fiducial markers. Alternatively, the gantry may be rotated by 90 degrees to acquire images at two different angles and create a stereoscopic view. However, since the two images would not be taken simultaneously, this method does not provide real-time stereoscopic imaging and therefore cannot be used for monitoring respiratory motion. Other developers have developed a real-time stereoscopic imaging modality for IGRT by mounting two kV x-ray source and FPI detector pairs on the floor and ceiling of the treatment room. Unfortunately, this method does not have the capability to perform volumetric CT imaging.
The current CBCT systems with one point source and one flat panel imager are not able to provide stereoscopic imaging functionality. Their fluoroscopic imaging function cannot detect motion along the kV beam direction. When the kV beam is orthogonal to the MV beam, this motion component can cause geometric miss of the target as shown in FIGS. 1A and 1B.